Birefringent networks

ABSTRACT

A birefringent network can be formed from a pair of beam directing elements that sandwich a birefringent filter. One of the beam director elements can be a polarizing beam splitter and the other of the beam director elements can be a reflector or polarizing beam splitter. The polarizing beam splitters can be formed by an anisotropic material sandwiched between two isotropic pieces and can include optical films to couple both polarizations of light into and out of the beam splitters. The polarizing beam splitters also can be formed by anisotropic material on either side of an isotropic bow-tie piece, all of which is sandwiched a pair of isotropic pieces. A birefringent network also can be formed by a pair of bulk birefringent beam splitters sandwiching a birefringent filter. Little or no polarization mode dispersion occurs in these birefringent networks since all of the beams travel the same distances through the same elements.

PRIORITY CLAIM AND RELATED APPLICATIONS

[0001] The present application claims priority from U.S. ProvisionalApplication entitled “Birefringent Networks,” Serial No. 60/407,033filed Aug. 30, 2002, having Jonathan R. Birge and Gary D. Sharp, asinventors, and having as assignee ColorLink, Inc., the assignee of thepresent application. This provisional application is incorporated hereinby reference in its entirety for all purposes.

FIELD OF THE INVENTION

[0002] The present invention relates generally to optical filters, theiruse and manufacture, and more particularly to birefringent networkshaving beam splitters and birefringent filters, their use andmanufacture.

BACKGROUND

[0003] Optical filters can be advantageously combined with varioustechnologies. One such area is telecommunications. Telecommunicationoptical filters typically require minimal insertion loss andpolarization dependent loss (PDL). Various polarization diversitytechniques have been employed to achieve minimal insertion loss and PDLfor birefringent filters.

[0004] Polarization diversity techniques split the light input into theoptical filter into its orthogonal components for filtering. Thefiltered light may then be recombined and output as an unpolarized beam.

[0005] Some prior art polarization diversity techniques have used cubepolarization beam splitters to split the input light and to recombinethe filtered light. Unfortunately, cube polarization beam splitters havelarge beam spacing, must be precisely aligned, and do not always providehigh extinction ratios.

[0006] Other prior art polarization diversity techniques have usedwalk-off configurations with expensive bulk birefringent materials suchas calcite or yttrium vannadate. The walk-off configurations requirecomplex optical paths when using both outputs in a polarizationinsensitive manner. This results in an expensive and complexarchitecture.

[0007] Polarization mode dispersion (PMD) has become increasinglyimportant as optical data rates have increased. Complex architectureshave additional delay symmetry elements to compensate for PMD. Simplerarchitecture can also compensate for PMD. The simpler architecturerequires the use of extra compensating elements of precise thickness.Unfortunately, the asymmetric architecture of these PMD compensateddevices renders them sensitive to temperature changes.

[0008] Accordingly, there is a strong need for an inexpensive opticalfilter with a small beam spacing and simple optical path that has littleor negligible PMD.

SUMMARY OF THE INVENTION

[0009] An aspect of the invention is to provide a birefringent networkincluding a first beam director that redirects light into a first pathand a second path, a second director that receives light directed alongthe first and second paths, and a birefringent filter between the firstand second beam directors that filters the light which traverses alongthe first and second paths. The length of the first path issubstantially equal to the length of the second path, and the opticallength of the first path is substantially equal to the optical length ofthe second path.

[0010] Another aspect of the invention is to provide a birefringentnetwork including a first beam director that redirects light into afirst path and a second path, the first beam director including anisotropic/anisotropic planar interface that transmits light of onepolarization and reflects light of an orthogonal polarization, a secondbeam director that receives light directed along the first and secondpaths; and a birefringent filter between the first and second beamdirectors that filters the light which traverses along the first andsecond paths. The first and second paths from the isotropic/anisotropicplanar interface to the second director are symmetric about theisotropic/anisotropic planar interface.

[0011] Another aspect of the invention is to provide a film for couplingplural polarization components of incident light between two mediahaving different refractive indices including a media, and a filmlocated at a surface of the media. The film couples at least twopredetermined orthogonal polarization components of an acute incidentlight into the media.

[0012] Another aspect of the invention is to provide a beam directorincluding first and second isotropic media sandwiching an anisotropicmedia along a sandwiching direction. The light input into and outputfrom the beam director are parallel to the sandwiching direction.

[0013] Another aspect of the invention is to provide a method ofaffecting light with a birefringent network including redirecting lightinto a first path and a second path with a first beam director,receiving light directed along the first and second paths into a seconddirector, and filtering the light which traverses along the first andsecond paths that is between the first and second beam directors with abirefringent filter. The length of the first path is substantially equalto the length of the second path, and the optical length of the firstpath is substantially equal to the optical length of the second path.

[0014] Another aspect of the invention is to provide a method ofaffecting light with a birefringent network including redirecting lightinto a first path and a second path with a first beam director, thefirst beam director including an isotropic/anisotropic planar interfacethat transmits light of one polarization and reflects light of anorthogonal polarization, receiving light directed along the first andsecond paths with a second beam director, and filtering the light whichtraverses along the first and second paths between the first and secondbeam directors with a birefringent filter. The first and second pathsfrom the isotropic/anisotropic planar interface to the second directorare symmetric about the isotropic/anisotropic planar interface.

[0015] Another aspect of the invention is to provide a method forcoupling plural polarization components of incident light between twomedia having different refractive indices with a film includingproviding a media, and forming a film located at a surface of the mediasuch that the film couples at least two predetermined orthogonalpolarization components of an acute incident light into the media.

[0016] Another aspect of the invention is to provide a method ofaffecting light with a beam director including providing first andsecond isotropic media sandwiching an anisotropic media along asandwiching direction, inputting input light in a direction parallel tothe sandwiching direction, and outputting output light in a directionparallel to the sandwiching direction.

[0017] Another aspect of the invention is to provide a method ofaffecting light with a birefringent network including providing a firstand second bulk birefringent beam splitter, separating input light intoa first light beam and a second light beam with the first bulkbirefringent beam splitter, filtering the first and second light beamswith a birefringent filter sandwiched between the first and second bulkbirefringent beam splitters, separating the filtered first light beamsinto third and fourth light beams with the second bulk birefringent beamsplitter, separating the filtered second light beam into fifth and sixthlight beams with the second bulk birefringent beam splitter, combiningthe fourth and fifth light beams into a combined beam. The third andsixth light beams are polarized light beams, and the combined light beamis an unpolarized light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 shows a block diagram of a birefringent network;

[0019]FIG. 2 shows an embodiment of a polarizing beam splitter of thepresent invention;

[0020]FIG. 3 shows another embodiment of polarizing beam splitter of thepresent invention;

[0021]FIG. 4 shows a cube reflector;

[0022]FIG. 5 shows a three-port birefringent network;

[0023]FIG. 6 shows an add-drop birefringent network;

[0024]FIG. 7 shows a three-port birefringent network includingachromatic compensation elements;

[0025]FIG. 8 shows a three port birefringent network with an activeswitching element;

[0026]FIG. 9 shows a three-port birefringent network includingachromatic compensation elements with an active switching element;

[0027]FIG. 10 shows a three-port birefringent network includingachromatic compensation elements and an active switching element;

[0028]FIG. 11 shows a two-port birefringent network using the beamdirectors of FIG. 3;

[0029]FIG. 12 shows a two-port birefringent network of FIG. 11 withachromatic compensation elements;

[0030]FIG. 13 is a plot of a computer simulation of a gain flatteningfilter implemented with a two-port birefringent network;

[0031]FIG. 14 is a plot of a simulation of a broadband filterimplemented with a three-port birefringent network; and

[0032]FIG. 15 shows a walk-off configured birefringent network.

DETAILED DESCRIPTION

[0033]FIG. 1 shows a block diagram of a birefringent network 100. Afirst beam director 102, a second beam director 104 and a birefringentfilter 106 are the three basic components that form the birefringentnetwork 100. An unpolarized light beam 108 is separated into twoorthogonally polarized light beams 110A, 110B by the first beam director102. The two orthogonally polarized light beams 110A, 110B then arefiltered by the birefringent filter 106 to produce two filtered lightbeams 112A, 112B. The filtered light beams 112A, 112B are then inputinto the second beam director 104. The second beam director 104 causesone or both of the filtered light beams 112A, 112B to be directed into afirst output beam 114 or second output beam 116, or to be directed backtowards the birefringent filter 106. Optionally, another unpolarizedlight beam 118 may also be input into the birefringent network 100. Thisanother unpolarized light beam 118 will be processed in a manner similarto that of the unpolarized light beam 108. Where the unpolarized lightbeams 108, 118 are directed is determined by the configuration of thebirefringent network 100, the characteristics of the unpolarized lightbeams 108, 118 input into the birefringent network 100 and the point ofentry for the unpolarized light beams 108, 118. The birefringent filter106 may be any appropriate kind of birefringent filter.

[0034]FIG. 2 shows an embodiment of polarizing beam splitter 120 of thepresent invention that may be used as either one or both of the beamdirectors 102, 104 of FIG. 1. The polarizing beam splitter 120 includesa first isotropic piece 122A and a second isotropic piece 122B thatsandwiches a layer of anisotropic material 124. The isotropic pieces122A, 122B (or isotropic prisms since they refract light) can be madefrom any isotropic material including glass and plastic. The anisotropiclayer 124 can be made of any anisotropic material provided the isotropicpieces 122A, 122B and the anisotropic layer 124 function to separatelight beams 108, 118 into two light beams 110A, 110B of orthogonalpolarization. For example, the anisotropic layer 124 can be a highlybirefringent nematic liquid crystal with homeotropic alignment in azero-twist configuration. The molecular directors should all be alignednormal to the light beams 108, 118 and parallel to the interface betweenthe isotropic and anisotropic materials (i.e., the molecular directorsshould be aligned parallel to the cross of the light beams). Theextraordinary index of refraction of the liquid crystal (n_(e)) shouldmatch the index of refraction of the isotropic pieces (n_(iso)) in orderto allow transmission of the s-polarization portion of the light beams108, 118 with minimal Fresnel loss. Total internal reflection of thep-polarization portion of the light beams 108, 118 occurs due to theacute angle of incidence of the light beams 108, 118 and the largerefractive index mismatch between n_(iso) and the ordinary index ofrefraction of the liquid crystal (n_(o)). Alternatively, thep-polarization portion could be transmitted and the s-polarizationportion reflected by matching n_(o) and n_(iso) where n_(e)<n_(o)instead of matching n_(e) and n_(iso). In either case, the thickness ofthe anisotropic layer 124 should be large enough to avoid tunneling ofthe light of the polarization that is to be reflected.

[0035] Optical films 126 can be formed at the input and output points ofthe polarization beam splitter 120. These optical films 126 ensure thesubstantial coupling of both the s and p polarized light components intoand out of the polarizing beam splitter 120. Thus, enabling equalportions of s and p polarized light to be coupled in and out of thepolarizing beam splitter 120 at larger acute angles (e.g., about 49°).The thickness and index of refraction of the optical films 126 can bedetermined using a constrained least squares optimization method oranother method. The optical films can be formed using conventionalanti-reflection film deposition techniques.

[0036] Additionally, the interfaces between the isotropic pieces 122A,122B and the anisotropic material 124 could include an opticaltransition film (not shown) when the n_(iso), cannot be exactly matchedto an index of refraction of the anisotropic layer 124. The opticaltransition film would have an index of refraction selected to bettertransmit the light of the polarization to be transmitted (e.g., theindex of refraction of the transition film being between n_(iso) and the“matched” index of refraction of the anisotropic layer 124). The opticaltransition film also must continue to reflect light of the polarizationto be reflected (e.g., the difference between the index of refraction ofthe transition film and the “unmatched” index of refraction beingsufficiently large). Alternatively, the transition film could be formedfrom multiple layers instead of a single layer.

[0037]FIG. 3 shows another embodiment of polarizing beam splitter 120′of the present invention that may be used as either one or both of thebeam directors 102, 104 of FIG. 1. The polarizing beam splitter 120′ hastwo isotropic/anisotropic interfaces. The first isotropic/anisotropicinterface includes a first isotropic piece 122A and an isotropic bow-tieportion 122C that sandwiches anisotropic material 124A along two sidesof the first isotropic piece 122A. The second isotropic/anisotropicinterface includes a second isotropic piece 122B and the isotropicbow-tie portion 122C that sandwiches anisotropic material 124B along twosides of the second isotropic piece 122B. The isotropic pieces 122A,122B and the bow-tie portion 122C can be made from any isotropicmaterial including glass and plastic. The anisotropic material 124A,124B can be made of any isotropic material provided the isotropic pieces122A, 122B and the isotropic bow-tie portion 122C in combination withthe anisotropic material 124A, 124B functions to separate the lightbeams 108, 118 into two light beams 110A, 110B of orthogonalpolarization. For example, the anisotropic material 124A, 124B can be ahighly birefringent nematic liquid crystal with homeotropic alignment ina zero-twist configuration. The molecular directors should all bealigned normal to the light beams 108, 118 and parallel to theinterfaces between the isotropic and anisotropic materials (i.e., themolecular directors should be aligned parallel to the cross of the lightbeams). The extraordinary index of refraction of the liquid crystal(n_(e)) should match the index of refraction of the isotropic pieces andbow-tie portion (n_(iso)) in order to allow transmission of thes-polarization portion of the light beams 108, 118 with minimal Fresnelloss. Total internal reflection of the p-polarization portion of thelight beams 108, 118 occurs due to the acute angle of incidence of thelight beams 108, 118 and the large refractive index mismatch betweenn_(iso) and the ordinary index of refraction of the liquid crystal(n_(o)). Alternatively, the p-polarization portion could be transmittedand the s-polarization portion reflected by matching n_(o) and n_(iso)instead of matching n_(e) and n_(iso). In either case, the thickness ofthe anisotropic layer 124 should be large enough to avoid tunneling ofthe light of the polarization that is to be reflected. The isolationbetween orthogonal polarization components for the polarization beamsplitter 120′ of FIG. 3 is approximately twice that of the polarizationbeam splitter 120 of FIG. 2 because the light beams 108, 118 must passthrough or be reflected twice during transit through the polarizing beamsplitter 120′.

[0038] Additionally, the interfaces between the anisotropic material124A, 124B and any of the isotropic pieces 122A, 122B and the isotropicbow-tie portion 124C could include an optical transition film (notshown) when the n_(iso) cannot be exactly matched to an index ofrefraction of the anisotropic material 124A, 124B. The opticaltransition film would have an index of refraction selected to bettertransmit the light of the polarization to be transmitted (e.g., theindex of refraction of the transition film being between n_(iso) and the“matched” index of refraction of the anisotropic layer 124). The opticaltransition film also must continue to reflect light of the polarizationto be reflected (e.g., the difference between the index of refraction ofthe transition film and the “unmatched” index of refraction beingsufficiently large). Alternatively, the transition film could be formedfrom multiple layers instead of a single layer. The light throughput maybe further increase by including conventional anti-reflective films atthe input and exit surfaces of the polarizing beam splitter 120′.

[0039]FIG. 4 shows a cube reflector 128 that may be used as the secondbeam director 104 of FIG. 1. The cube reflector 128 reflects receivedlight from one path and reflects it along the other path. A plane mirroror a retro-reflector may be used to reflect received light back alongthe same path. Other types of reflectors may also be used as the secondbeam director 104 and the reflector may also be couple with otherelements.

[0040]FIG. 5 shows a three-port birefringent network 200 according tothe present invention. The three-port network 200 includes a firstpolarizing beam splitter 202, a second polarizing beam splitter 204 anda birefringent filter 206. The three-port birefringent network 200 canbe used to separate a COMMON light beam 208 that is input into thethree-port birefringent network 200 into a first light beam 210 with onesub-set of frequencies and a second light beam 212 with another sub-setof frequencies. Conversely, the three-port birefringent network 200 canbe used to combine a first light beam 208 with one sub-set offrequencies and a second light beam 212 with another sub-set offrequencies into a COMMON light beam 200 by reversing the direction oflight flow.

[0041] The COMMON light beam 208 enters the first beam splitter 202parallel to the plane of its anisotropic layer 124 and the two separatedand polarized components of the COMMON light beam 208 exit parallel tothe COMMON light beam 208. The two components remain parallel to theCOMMON light beam 208 during transit through the birefringent filter 206and enter the second beam splitter 204 parallel its anisotropic layer124. The first and second light beams 210, 212 are then output fromsecond beam splitter 204 parallel to the COMMON light beam 208.Accordingly, the light that enters and exits the polarizing beamsplitters 202, 204 is all parallel. Additionally, the anisotropic layers124 of the polarizing beam splitters 202, 204 also are parallel to thelight that enters and exits the polarizing beam splitters 202, 204.

[0042]FIG. 6 shows an add-drop birefringent network 214 according to thepresent invention. The add-drop birefringent network 214 is structurallythe same as the three-port birefringent network 200 of FIG. 5 except forthe inclusion of another input light beam. The add-drop birefringentnetwork 214 can be used to separate out a sub-set of frequencies from aninput light beam 216 into a DROP light beam 218. The birefringentnetwork 214 then combines the sub-set of frequencies that remain fromthe INPUT light beam 216 with an ADD light beam 220 to form a THRU lightbeam 222.

[0043] The INPUT light beam 216 enters the first beam splitter 202parallel to the plane of its anisotropic layer 124 and the two separatedand polarized components of the INPUT light beam 216 exit parallel tothe INPUT light beam 216. The two components remain parallel to theINPUT light beam 216 during transit through the birefringent filter 206and enter the second beam splitter 204 parallel its anisotropic layer124. The ADD and DROP light beams 218, 222 are then output from secondbeam splitter 204 parallel to the INPUT light beam 216.

[0044] Similarly, the ADD light beam 220 enters the first beam splitter202 parallel to the plane of its anisotropic layer 124 and the twoseparated and polarized components of the ADD light beam 220 exitparallel to the ADD light beam 220. The two components remain parallelto the ADD light beam 220 during transit through the birefringent filter206 and enter the second beam splitter 204 parallel its anisotropiclayer 124. The ADD and DROP light beams 218, 222 are then output fromsecond beam splitter 204 parallel to the INPUT light beam 216.Accordingly, the light that enters and exits the polarizing beamsplitters 202, 204 is all parallel. Additionally, the anisotropic layers124 of the polarizing beam splitters 202, 204 also are parallel to thelight that enters and exits the polarizing beam splitters 202, 204.

[0045]FIG. 7 shows a three-port birefringent network 224 including firstand second achromatic compensation elements 226, 228. The three-portbirefringent network 224 of FIG. 7 is structurally the same as thethree-port birefringent network 200 of FIG. 5 except for the addition ofthe two achromatic compensation elements 226, 228. The first achromaticcompensation element 226 is placed in a first path on one side of thebirefringent filter 206 and the second achromatic compensation element228 is placed in a second path on the other side of the birefringentfilter 208. Each achromatic compensation element 226, 228 is formed froma stack of birefringent plates. The birefringent plates are oriented andhave an optical thickness (Δn·d) designed to produce an achromaticrotation of 90° (an achromatic half waveplate). The 90° rotationintroduced by the first achromatic compensation element 226 20 changesthe orthogonal polarization relationship of the light beams output bythe first beam splitter 202 into a parallel polarization relationship.The parallel relationship of the light input into the birefringentfilter 206 ensures that the same group delay affects both light beamsand no PMD dispersion. The second compensation element 228 recreates theorthogonal polarization relationship between the two light beams andequalizes the optical path lengths of the first and second paths. Thepolarization reversal causes the output from the second polarizationbeam splitter 204 to act in a reversed manner (i.e., light that wouldhave been reflected is now transmitted and light that would have beentransmitted is not reflected).

[0046] FIGS. 8-10 essentially are the birefringent networks of FIGS. 5-7with active switching. The active switching is performed by an activeswitching element 134 such as a liquid crystal cell. The activeswitching element 134 can provide 90° of rotation to the light in thefirst and second paths simultaneously (i.e., a single pixel activeswitching element) or the active switching element 134 can provide 90°of rotation to the light in one or both of the first and second paths(i.e., a two pixel active switching element). The single pixel activeswitching element is advantageous in that it has a simple structurewhile the two pixel active switching element provides independentcontrol over the light in each path.

[0047] In FIGS. 8 and 10, the single pixel embodiment of the activeswitching element 134 enables the transposition of light directed intothe first and second light beams 210, 212. The two pixel embodiment ofthe active switching element 134 enables the independent control oflight directed into the first and second beams 210, 212. Specifically,the active birefringent network directs the light normally when bothpixels are “off” (no rotation of the polarization of the light). All ofthe light of one polarization is directed to one of the two light paths210, 212 and all of the light of the orthogonal polarization is directedto the other of the two light paths 210, 212 when one pixel is turned“on” (a half wave rotation of the polarization of the light) and onepixel is turned “off”. Reversing which pixel is “on” and which pixel is“off” reverses which polarization is directed into each of the two lightbeams 210, 212. The light beam to which light is directed is transposedas compare to a passive device when both pixels are “on”. However, FIGS.8 and 10 differ in that the achromatic compensation elements 226, 228 ofFIG. 10 reverse the polarizations that are output into the first andsecond light beams 210, 212 as compare to FIG. 8.

[0048] In FIG. 9., the single pixel embodiment of the active switchingelement 134 enables the transposition of light directed into the DROPand THRU light beams 218, 222. The two pixel embodiment of the activeswitching element 134 enables the independent control of light directedinto the DROP and THRU light beams 218, 222. Specifically, the activebirefringent network directs the light normally when both pixels are“off” (no rotation of the polarization of the light). All of the lightof one polarization from the INPUT light beam 216 is directed to one ofthe DROP and THRU light paths 218, 222 and all of the light of theorthogonal polarization from the INPUT light beam 216 is directed to theother of the DROP and THRU light paths 218, 222 when one pixel is turned“on” (a half wave rotation of the polarization of the light) and onepixel is turned “off”. All of the light of the orthogonal polarizationfrom the ADD light beam 220 is directed to the one of the DROP and THRUlight paths 218, 222 and all of the light of the one polarization fromthe ADD light beam 220 is directed to the other of the DROP and THRUlight paths 218, 222 when one pixel is turned “on” (a half wave rotationof the polarization of the light) and one pixel is turned “off”.Reversing which pixel is “on” and which pixel is “off” reverses whichpolarizations are directed into the DROP and THRU light paths 218, 222.The light path to which light is directed is transposed as compare topassive device when both pixels are “on”.

[0049]FIG. 11 shows a two-port birefringent network 300 according to thepresent invention. The two-port network 300 includes a first polarizingbeam splitter 302, a second polarizing beam splitter 304 and abirefringent filter 206. The reflected light travels farther than thetransmitted light in each of the polarizing beam splitters 302 becauseit is reflected twice. This results in an asymmetry in the birefringentnetwork 300 that is not present in the birefringent networks of FIGS.5-10. Thus, the second polarizing beam splitter 304 is selected tointernally reflect light transmitted through polarizing beam splitter302 and to transmit light reflected in polarizing beam splitter 302(e.g., the first polarizing beam splitter 302 transmits incidents-polarized light and twice reflects incident p-polarized light whilethe second polarizing beam splitter 304 transmits incident p-polarizedlight and twice reflects incident s-polarized light). The opposingasymmetries of the two polarizing beam splitters 302, 304 cause allinput light 306 traveling through the two-port birefringent network 300to traverse equal the path lengths before being output as filtered light308. It is also possible to make both polarizing beam splitters 302, 304of identical construction by including an achromatic half waveplate (notshown) between the polarizing beam splitters 302, 304.

[0050]FIG. 12 shows a two-port birefringent network 310 according to thepresent invention that includes achromatic compensation. The two-portnetwork 310 includes all of the elements of the two-port network 300 ofFIG. 11 and additionally includes a first achromatic compensationelement 226 and a second achromatic compensation element 228. Theachromatic elements 226, 228 function in the manner described above withregard to FIG. 7. However, the 90° rotation introduced by the achromaticelements 226, 228 advantageously allows both polarization beam splitters302, 304 to be of identical construction without additional elements.

[0051]FIG. 13 is a plot of a computer simulation of a gain flatteningfilter implemented with a two-port birefringent network. The gainflattening filter helps equalize the amount of power in each channeland/or frequency. The two-port birefringent network is constructedsubstantially as shown in FIG. 5.

[0052]FIG. 14 is a plot of a simulation of a broadband filterimplemented with a three-port birefringent network. The three-portbirefringent network is constructed substantially as shown in FIG. 5.

[0053]FIG. 15 shows a walk-off configured birefringent network 400according to the present invention. The walk-off configured birefringentnetwork 400 includes a first bulk birefringent beam splitter 402, asecond bulk birefringent beam splitter 404 and a birefringent filter206. The first bulk birefringent beam splitter 402 divides an input beam406 into two light beams 408, 410 of orthogonal orientations that arethen filtered by the birefringent filter 206. The filtered light beams412, 414 are each divided into two separate light beams. The top lightbeam 416 travels farther than any other light beam because it iswalked-off twice. The bottom light beam 418 travels less distance thanany other light beam because it is never walked-off. The middle lightbeam 420 is composed of parts of both of the light beams 408, 410 outputfrom the first bulk birefringent. The light of the middle light beam 420has all traveled about the same distance since both components of themiddle light beam 420 are walked-off once. (The first bulk birefringentbeam splitter 402 has the same thickness as the second bulk birefringentbeam splitter 404.) The top light beam 416 and the bottom light beam areboth polarized and the middle light beam 414 is unpolarized. The simplegeometry and small spacing of the light beams reduces the cost becausesmaller bulk polarization beam splitters may be used.

[0054] The birefringent networks of the present invention may beconstructed as various kinds of single stage two-port, three-port andfour-port birefringent networks. For example, the birefringent networksdiscussed herein can be used as wavelength division multiplexers,interleavers, dispersion compensators, wavelength selective switches(e.g., 2×2) or rotators. The birefringent networks of the presentinvention also may be cascaded to form more complex devices. Thebirefringent filters described herein can be any kind of birefringentfilter. For example, the birefringent filters could be retarder stacks,bulk birefringent materials or liquid crystal devices.

[0055] As used herein, the word “redirect”, “redirected” and“redirecting” should be understood to be distinguished from “transmit”,“transmitted” and “transmitting”. For example, light normally incidentupon a cube beam splitter separates the light into two orthogonal beams.One beam is reflected perpendicular to the incident light direction andthe other beam continues to travel parallel to the incident lightdirection. Both beams are transmitted though the cube beam splitter butonly the reflected beam is redirected by the beam splitter. The cause(e.g., reflection, refraction, etc.) of the redirection is not relevantto the term. These terms should otherwise be broadly interpreted.

[0056] Although several embodiments of the present invention and itsadvantages have been described in detail, it should be understood thatchanges, substitutions, transformations, modifications, variations,permutations and alterations may be made therein without departing fromthe teachings of the present invention, the spirit and the scope of theinvention being set forth by the appended claims.

We claim:
 1. A birefringent network comprising: a first beam directorthat redirects light into a first path and a second path; a seconddirector that receives light directed along the first and second paths;and a birefringent filter between the first and second beam directorsthat filters the light which traverses along the first and second paths;wherein the length of the first path is substantially equal to thelength of the second path; and wherein the optical length of the firstpath is substantially equal to the optical length of the second path. 2.The network of claim 1, wherein: the first beam director includes amedia and a film located at a surface of the media; and the film couplesat least two predetermined orthogonal polarization components of anacute incident light into the media.
 3. The network of claim 1, whereina film thickness and a film index of refraction of the film are selectedto couple substantial portions of the at least two predeterminedorthogonal polarization components into the media.
 4. The network ofclaim 1, wherein the film has at least two layers, each layer having athickness and refractive index selected couple substantial portions ofthe at least two predetermined orthogonal polarization components intothe media.
 5. The network of claim 1, wherein the first director is apolarizing beam splitter.
 6. The network of claim 5, wherein thepolarizing beam splitter is part of a birefringent network.
 7. Thenetwork of claim 6, wherein the birefringent network is one of awavelength division modulator, an interleaver, a dispersion compensator,and a switch.
 8. The network of claim 6, wherein the birefringentnetwork is 2×2 switch.
 9. The network of claim 1, wherein light inputinto the birefringent network travels parallel to light output by thebirefringent network.
 10. The network of claim 9, wherein lighttravelling through the birefringent filter is parallel to the lightinput into the birefringent network.
 11. The network of claim 1, whereinlight is incident upon the first beam director at two different points.12. The network of claim 11, wherein light output by the second beamdirector is output at two different points.
 13. The network of claim 1,wherein light output by the second beam director is output at twodifferent points.
 14. The network of claim 1, further comprising: afirst achromatic compensation element in the first path; and a secondachromatic compensation element in the second path.
 15. The network ofclaim 1, further comprising an active element between the first beamdirector and the second beam director for selectively altering at leastone characteristic of light.
 16. The network of claim 15, wherein theactive element can simultaneously alter the at least one characteristicof light for the light traveling along the first and second paths. 17.The network of claim 15, wherein the active element can independentlyalter the at least one characteristic of light for the light travelingalong each of the first and second paths.
 18. The network of claim 1,wherein the first and second beam directors have the same configuration.19. The network of claim 18, wherein each beam director comprises firstand second isotropic media sandwiching an anisotropic media.
 20. Thenetwork of claim 19, wherein the anisotropic media allows transmissionof light of one polarization and causes reflection of light of anorthogonal orientation to the one polarization.
 21. The network of claim19, wherein the anisotropic media is a homeotropically alignedzero-twist nematic liquid crystal.
 22. The network of claim 18, whereineach beam director includes first and second isotropic media sandwichingan anisotropic media along a sandwiching direction such that light inputinto and output from each beam director are parallel to the sandwichingdirection.
 23. The network of claim 1, wherein light received by thesecond beam director from the first and second paths is caused to beoutput from the second beam director back towards the first beamdirector along the first and second paths.
 24. The network of claim 23,wherein the second beam director is a reflector.
 25. The network ofclaim 1, wherein the first beam director, the second beam director andthe birefringent filter are a passive network.
 26. The network of claim25, wherein the passive network is able to separate a predeterminedoptical signal from a plurality of optical signals contained withinlight received by the passive network.
 27. The network of claim 25,wherein the passive network is able to integrate an optical signal intolight output by the passive network.
 28. The network of claim 25,wherein: the passive network is able to separate a desired opticalsignal from a plurality of optical signals contained within lightreceived by the passive network; and the passive network is able tointegrate a different optical signal into light output by the passivenetwork.
 29. The network of claim 1, wherein: the first beam directorincludes two first isotropic/anisotropic interfaces, the firstisotropic/anisotropic interfaces transmitting light of one polarizationstate and reflecting light of an orthogonal polarization state; and thesecond beam director includes two first isotropic/anisotropicinterfaces, the first isotropic/anisotropic interfaces transmittinglight of the orthogonal polarization state and reflecting light of theone polarization state.
 30. A birefringent network comprising: a firstbeam director that redirects light into a first path and a second path,the first beam director including an isotropic/anisotropic planarinterface that transmits light of one polarization and reflects light ofan orthogonal polarization; a second beam director that receives lightdirected along the first and second paths; and a birefringent filterbetween the first and second beam directors that filters the light whichtraverses along the first and second paths; wherein the first and secondpaths from the isotropic/anisotropic planar interface to the seconddirector are symmetric about the isotropic/anisotropic planar interface.31. A film for coupling plural polarization components of incident lightbetween two media having different refractive indices comprising: amedia; and a film located at a surface of the media; wherein the filmcouples at least two predetermined orthogonal polarization components ofan acute incident light into the media.
 32. The film of claim 31,wherein a film thickness and a film index of refraction are selected tocouple substantial portions of the at least two predetermined orthogonalpolarization components into the media.
 33. The film of claim 31,wherein the film has at least two layers, each layer having a thicknessand refractive index selected to couple substantial portions of the atleast two predetermined orthogonal polarization components into themedia.
 34. The film of claim 31, wherein the media is part of apolarizing beam splitter.
 35. The film of claim 34, wherein polarizingbeam splitter is part of a birefringent network.
 36. The film of claim35, wherein the birefringent network is one of a wavelength divisionmodulator, an interleaver, a dispersion compensator, and a switch. 37.The film of claim 35, wherein the birefringent network is 2×2 switch.38. The film of claim 31, wherein the acute angle is about 49°.
 39. Abeam director comprising: first and second isotropic media sandwichingan anisotropic media along a sandwiching direction; wherein light inputinto and output from the beam director are parallel to the sandwichingdirection.
 40. The beam director of claim 39, wherein: the light inputinto the beam director is unpolarized and the light output from the beamdirector is separated into two orthogonally polarized light beams. 41.The beam director of claim 40, wherein the two orthogonally polarizedlight beams are separated by a lateral distance that is substantiallyless than a lateral thickness of the beam director.
 42. The beamdirector of claim 40, wherein the two orthogonally polarized light beamsare separated by short distance.
 43. The beam director of claim 40,wherein the first and second isotropic media are glass or plastic. 44.The beam director of claim 40, wherein light input area of the firstisotropic media is coated with an optical film that assists the couplingat least two predetermined orthogonal polarization components of anacute incident light into the media.
 45. A method of affecting lightwith a birefringent network comprising: redirecting light into a firstpath and a second path with a first beam director; receiving lightdirected along the first and second paths into a second director; andfiltering the light which traverses along the first and second pathsthat is between the first and second beam directors with a birefringentfilter; wherein the length of the first path is substantially equal tothe length of the second path; and wherein the optical length of thefirst path is substantially equal to the optical length of the secondpath.
 46. The method of claim 45, wherein: the first beam directorincludes a media and a film located at a surface of the media; and thefilm couples at least two predetermined orthogonal polarizationcomponents of an acute incident light into the media.
 47. The method ofclaim 45, further comprising coupling substantial portions of the atleast two predetermined orthogonal polarization components into themedia by selection a film thickness and a film index of refraction. 48.The method of claim 45, further comprising coupling substantial portionsof the at least two predetermined orthogonal polarization componentsinto the media by selection of film thicknesses and film indexes ofrefraction.
 49. The method of claim 45, wherein the first director is apolarizing beam splitter.
 50. The method of claim 49, wherein thepolarizing beam splitter is part of a birefringent network.
 51. Themethod of claim 50, wherein the birefringent network performs one ofwavelength division modulation, interleavering, dispersion compensating,and switching.
 52. The method of claim 50, wherein the birefringentnetwork performs 2×2 switching.
 53. The method of claim 45, whereinlight input into the birefringent network travels parallel to lightoutput by the birefringent network.
 54. The method of claim 53, whereinlight travelling through the birefringent filter is parallel to thelight input into the birefringent network.
 55. The method of claim 45,wherein light is incident upon the first beam director at two differentpoints.
 56. The method of claim 55, wherein light output by the secondbeam director is output at two different points.
 57. The method of claim45, wherein light output by the second beam director is output at twodifferent points.
 58. The method of claim 45, further comprising:providing a first achromatic compensation for the first path; andproviding a second achromatic compensation for the second path.
 59. Themethod of claim 45, further comprising selectively altering at least onecharacteristic of light between the first beam director and the secondbeam director.
 60. The method of claim 59, wherein the selectivelyaltering can simultaneously alter the at least one characteristic oflight for the light traveling along the first and second paths.
 61. Themethod of claim 59, wherein the selectively altering can independentlyalter the at least one characteristic of light for the light travelingalong each of the first and second paths.
 62. The method of claim 45,wherein the first and second beam directors have the same configuration.63. The method of claim 62, wherein each beam director comprises firstand second isotropic media sandwiching an anisotropic media.
 64. Themethod of claim 63, wherein the anisotropic media allows transmission oflight of one polarization and causes reflection of light of anorthogonal orientation to the one polarization.
 65. The method of claim63, wherein the anisotropic media is a homeotropically alignedzero-twist nematic liquid crystal.
 66. The method of claim 62, whereineach beam director includes first and second isotropic media sandwichingan anisotropic media along a sandwiching direction such that light inputinto and output from each beam director are parallel to the sandwichingdirection.
 67. The method of claim 45, wherein light received by thesecond beam director from the first and second paths is caused to beoutput from the second beam director back towards the first beamdirector along the first and second paths.
 68. The method of claim 67,wherein the second beam director is a reflector.
 69. The method of claim45, wherein the first beam director, the second beam director and thebirefringent filter are a passive network.
 70. The method of claim 69,further comprising separating a predetermined optical signal from aplurality of optical signals contained within light received by thepassive network.
 71. The method of claim 69, further comprisingintegrating an optical signal into light output by the passive network.72. The method of claim 69, further comprising: separating a desiredoptical signal from a plurality of optical signals contained withinlight received by the passive network; and integrating a differentoptical signal into light output by the passive network.
 73. The methodof claim 45, wherein: the first beam director includes two firstisotropic/anisotropic interfaces, the first isotropic/anisotropicinterfaces transmitting light of one polarization state and reflectinglight of an orthogonal polarization state; and the second beam directorincludes two first isotropic/anisotropic interfaces, the firstisotropic/anisotropic interfaces transmitting light of the orthogonalpolarization state and reflecting light of the one polarization state.74. A method of affecting light with a birefringent network comprising:redirecting light into a first path and a second path with a first beamdirector, the first beam director including an isotropic/anisotropicplanar interface that transmits light of one polarization and reflectslight of an orthogonal polarization; receiving light directed along thefirst and second paths with a second beam director; and filtering thelight which traverses along the first and second paths between the firstand second beam directors with a birefringent filter; wherein the firstand second paths from the isotropic/anisotropic planar interface to thesecond director are symmetric about the isotropic/anisotropic planarinterface.
 75. A method for coupling plural polarization components ofincident light between two media having different refractive indiceswith a film comprising: providing a media; and forming a film located ata surface of the media such that the film couples at least twopredetermined orthogonal polarization components of an acute incidentlight into the media.
 76. The method of claim 75, wherein a filmthickness and a film index of refraction are selected to couplesubstantial portions of the at least two predetermined orthogonalpolarization components into the media.
 77. The method of claim 75,wherein the film has at least two layers, each layer having a thicknessand refractive index selected to couple substantial portions of the atleast two predetermined orthogonal polarization components into themedia.
 78. The method of claim 75, wherein the media is part of apolarizing beam splitter.
 79. The method of claim 78, wherein polarizingbeam splitter is part of a birefringent network.
 80. The method of claim79, wherein the birefringent network performs one of wavelength divisionmodulation, interleavering, dispersion compensating, and switching. 81.The method of claim 79, wherein the birefringent network performs 2×2switching.
 82. The method of claim 75, wherein the acute incidence angleis about 49°.
 83. A method of affecting light with a beam directorcomprising: providing first and second isotropic media sandwiching ananisotropic media along a sandwiching direction; inputting input lightin a direction parallel to the sandwiching direction; and outputtingoutput light in a direction parallel to the sandwiching direction. 84.The method of claim 83, further comprising separating the input lightinto two orthogonally polarized light beams, wherein the input light isunpolarized.
 85. The method of claim 84, wherein the two orthogonallypolarized light beams are separated by a lateral distance that issubstantially less than a lateral thickness of the beam director. 86.The method of claim 84, wherein the two orthogonally polarized lightbeams are separated by short distance.
 87. The method of claim 84,wherein the first and second isotropic media are glass or plastic. 88.The method of claim 84, wherein light input area of the first isotropicmedia is coated with an optical film that assists the coupling at leasttwo predetermined orthogonal polarization components of an acuteincident light into the media.
 89. A method of affecting light with abirefringent network comprising: providing a first and second bulkbirefringent beam splitter; separating input light into a first lightbeam and a second light beam with the first bulk birefringent beamsplitter; filtering the first and second light beams with a birefringentfilter sandwiched between the first and second bulk birefringent beamsplitters; separating the filtered first light beams into third andfourth light beams with the second bulk birefringent beam splitter;separating the filtered second light beam into fifth and sixth lightbeams with the second bulk birefringent beam splitter, combining thefourth and fifth light beams into a combined beam; wherein the third andsixth light beams are polarized light beams; and the combined light beamis an unpolarized light beam.